Increases in the density of components that can be placed on a semiconductor chip has been largely due to advances in photolithography. This has been associated with using radiation of ever decreasing wavelengths. As long as the minimum size (critical dimension or CD) of the components was greater than the wavelength of the radiation being used to expose the photoresist, advances in the art did not require any changes in the masks and optical systems used other than to reduce the sizes of the components.
When the critical dimension got to be less than about half the wavelength of the radiation being used radiation of lower wavelength had to be substituted. Eventually, critical dimensions reached, and then went below, the lower limit of optical lithography where conventional optics and resists can still be used.
When the wavelength of the imaging radiation gets to be less than the CD, the effects of diffraction, though always present, become prominent enough to introduce noticeable distortions into the projected images relative to their original shapes on the reticle. These distortions are particularly sensitive to the distances between the various features in the pattern and are therefore referred to as `proximity effects`. An entire branch of photolithography has been devoted to finding effective ways to deal with this problem. Among the solutions that have been adopted we may mention the use of serifs to compensate for the distortions, scattering bars that reduce the effective separation between lines without themselves appearing in the image, and phase shift masks that alter the optical path length near diffracting edges.
Although the above-noted solutions have been applied with some success, this success has been limited. Additionally, these techniques are time consuming as well as expensive and, in general, quite pattern dependent. There is thus a strong incentive to find ways to form images using radiation of shorter wavelength. Although it has been demonstrated that X-ray lithography is capable of producing patterns whose CD is one or two orders of magnitude less those obtainable with the best optical lithography, its cost remains unattractive at this time.
No matter how effective the above-mentioned compensation techniques, conventional optical lithography cannot be used below about 180 nm because the optical system becomes opaque to the radiation. The present invention shows how this limitation may be overcome by using two-photon absorption which we shall now discuss.
Harmonic light generation, or frequency doubling, has been known for a number of years. The basic principle is that when a suitable medium is irradiated with light of a given frequency most of the photons are absorbed, each such absorption causing an electron to be raised to a first energy level, and then re-emitted by spontaneous emission as the electron returns to its ground state. If the light intensity is sufficiently high, a second photon may be absorbed before spontaneous emission occurs, causing the electron to be raised to a new level whose energy is twice that of the first energy level. When this electron returns to the ground state, a photon having twice the frequency of the incoming radiation is emitted.
Although frequency doubling is, in principle, possible with any light source, given a suitable medium, in practice the necessary intensity is difficult to achieve with any sources other than lasers. It has also been found that the probability of frequency doubling can be significantly increased if the incoming radiation is in the form of very short but very intense pulses that arrive at high repetition rate. The high repetition rate increases the probability of a photon arriving at any given atom while the latter is still in an excited state while the high intensity of the pulse increases the number of photons per pulse. Furthermore, since the pulses are very narrow, the duty ratio remains relatively low, even at the high repetition rates, so that the overall power level of the incoming radiation can be quite low.
Although frequency doubling has been most widely observed in crystalline materials having suitable band structures, it can also occur in liquid or amorphous media provided they include a dye which is known to absorb the incoming radiation.
A routine search of the prior art was performed. References of interest that were found were U.S. Pat. No. 3,949,319 (Tofield) who shows a laser using multiple photon absorption. In U.S. Pat. No. 5,680,018, Yamada shows a method to generate a laser beam using multiple photon adsorption. Umstadter et al. in U.S. Pat. No. 5,637,966 shows a pulsed laser that is used to generates a plasma while Denk et al. in U.S. Pat. No. 5,034,613 describe two-photon laser microscopy.